1. Field of the Invention
The present invention relates to engine retarders of the compression release type. More particularly it relates to an improved control valve for a compression release engine retarder.
2. The Prior Art
Engine retarders of the compression release type are well known in the art. In general, such retarders are designed temporarily to convert an internal combustion engine into an air compressor so as to develop a retarding horsepower which may be a substantial portion of the operating horsepower normally developed by the engine in its powering mode.
The basic design for an engine retarding system of the type here involved is disclosed in the Cummins U.S. Pat. No. 3,220,392. In that design an hydraulic system is employed wherein the motion of a master piston actuated by an appropriate intake, exhaust or fuel injector pushtube or rocker arm controls the motion of a slave piston which opens the exhaust valve of the internal combustion engine near the end of the compression stroke whereby the work done in compressing the intake air is not recovered during the expansion or "power" stroke but, instead, is dissipated through the exhaust and cooling systems of the engine.
Various improvements have been made in the original design shown in the Cummins U.S. Pat. No. 3,220,392. Laas U.S. Pat. No. 3,405,699 discloses a device to unload the hydraulic system whenever excess motion of the slave piston tends to open the exhaust valve too far and hence risk damage to the components of the engine.
Sickler et al. U.S. Pat. No. 4,271,796 discloses a pressure relief system for a compression release engine retarder wherein a bi-stable ball relief valve and a damping mechanism rapidly drops the pressure in the hydraulic system to a predetermined low level whenever an excess pressure is sensed in the hydraulic system thereby obviating the risk of damage to various components in the engine valve train mechanism.
Custer U.S. Pat. No. 4,398,510 discloses an improved timing mechanism for an engine retarder which produces an increased retarding horsepower while increasing the time span between the beginning of the engine retarding action and the beginning of the normal opening of the exhaust valves of the engine.
Jakuba et al. U.S. Pat. No. 4,473,047 discloses a compression release engine retarder for an engine having dual exhaust valves wherein, during the retarding mode, only one of the dual exhaust valves is opened while in the powering mode both valves are opened.
Cavanagh U.S. Pat. No. 4,399,787 discloses an hydraulic reset mechanism particularly applicable to engine retarders of the type described in U.S. Pat. No. 4,473,047 wherein the exhaust valve opened during retarding is closed promptly after the retarding event has been completed and well before the normal opening of the dual exhaust valves begins thereby avoiding damage due to unbalanced or stress loading of the exhaust valve crosshead.
Despite the various improvements which have been made in the compression release retarder, including those noted above, certain problems still exist. During the retarding mode of operation high levels of pressure are experienced in the retarder hydraulic system. These high pressures act on the master piston, slave piston and control valve and result in leakage of oil past these elements. In order to reduce such leakage to an acceptable level, close tolerances must be maintained for each of these elements. It will be appreciated that if the control valve could be isolated from the high pressure hydraulic circuit, its manufacturing costs could be reduced substantially, its reliability improved, and the leakage of high pressure oil virtually eliminated. The elimination of such leakage would increase the retarding horsepower, render the retarding performance more consistent for each engine cylinder, and decrease the variation in performance among production models of engine retarders. Finally, the control valve would become less sensitive to contaminants in the engine oil supply used to operate the retarder. The present invention is directed to these objectives.